Abstract
The Laser Interferometer Gravitational Wave Observatory (LIGO) detect gravitational waves with large and precise Michelson interferometers. The gravitational wave signals from faraway stars are extremely weak so that it's necessary to reduce noise of the interferometer. The most sensitive observed frequency range of the interferometer is around 100Hz, and the sensitivity is majorly limited by coating Brownian noise of high reflective mirrors. According to the fluctuation-dissipation theorem, coating Brownian noise is proportional to mechanical loss of coating materials. Thus, the mechanical loss of coating materials is measured to evaluate coating Brownian noise, and it will be reduced by annealing. The first part of this thesis shows that the mechanical loss investigation of nano-layer structure consisting of alternating thin TiO2 and SiO2 films at room temperature. Several prototypes are designed, featuring a different number of TiO2/SiO2 layer pairs, and different thicknesses, but the same nominal refractive index. In several prototypes, the 19-layer structure is composed of thinnest TiO2/SiO2 layer, and exhibits highest threshold temperatures for crystallization onset. Therefore, the 19-layer is deposited by ion beam sputter and is annealed at 275oC for 24 hours. After annealing, the mechanical loss of the 19-layer at room temperature is effectively reduced from 1.16×10-3 to 1.28×10-4 around 100Hz. In the other part, coating Brownian noise is reduced in cryogenic. Cryogenic loss of the 19-layer structure is also investigated. In the results, measured losses are lower than the calculated loss from the individual TiO2/SiO2 layer. The cryogenic peak of SiO2 seems to be “covered out”, and the loss of the 19-layer is very close to that of the TiO2. After annealing, the cryogenic mechanical loss of the 19-layer is also reduced from 4.24×10-4 to 2.69×10-4 around 677Hz at 20 K. These result shows that annealing decreases mechanical loss of the 19-layer both at room temperature and cryogenic.